1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member and more particularly, to an electrophotographic photosensitive member suitable for laser printer.
2. Description of the Prior Art
Heretofore, as a photosensitive member for a printer of electrophotographic system using a coherent light represented by laser as a light source, there have been used selenium, selenium series alloys, cadmium sulfide dispersed in resin, charge transfer complex of polyvinyl carbazole and trinitrofluorenone and the like.
As a laser, there have been used gas lasers such as helium-cadmium, argon, helium-neon and the like. However, there have been recently used semiconductor lasers which are small, inexpensive, and can be directly modulated. However, the wavelength of emitted light of most semiconductor lasers is 750 nm or more, and conventional photosensitive members are of low photosensitivity at such wavelength range and are used with difficulty. Therefore, laminate type photosensitive members constituted of a charge transport layer and a charge generation layer which can relatively freely select the photosensitive wavelength region have recently drawn attention.
The charge generation layer of the laminate type photosensitive member absorbs light to generate free electric charge, and the thickness of the charge generation layer is usually as thin as 0.1-5 .mu.m so as to shorten the moving distance of the generated photo-carrier. The thickness of the charge generation layer is thin in order that most of the incident light may be absorbed in the charge generation layer to generate many photo-carriers and further the generated photo-carriers may be injected into the charge transport layer without being deactivated by recombination and trapping. As the charge transport layer, there is used a material capable of accepting electrostatic charge and transporting free electric charge and substantially incapable of absorbing an image forming light. The thickness of the charge transport layer is usually 5-30 .mu.m. When such a laminate type photosensitive member is used for forming images by line-scanning with a laser light by means of a laser printer, there can be produced good line images such as letters and the like, but when solid images are to be formed density irregularities, there are produced in the form of an interference fringe.
The formation of density irregularities seems to be attributable to a limited absorption of the light quantity in the charge generation layer, since the thickness of the layer is thin and the light having passed through the charge generation layer is reflected at the surface of the substrate and interference occurs between the resulting reflected light and the reflected light at the surface of the photoconductive layer.
A laminate type electrophotographic photosensitive member is constituted such that, as shown in FIG. 2, a charge generation layer 2 and a charge transport layer 3 are successively laminated to a metallic conductive support 1. When a laser beam 6 (the oscillating wavelength being about 780 nm in the case semiconductor laser and about 630 nm in the case of helium-neon laser) strikes the laminate type photosensitive member, there occurs interference between a light 7 indicent upon the charge transport layer 3 and a reflected light 8 resulting from another light 7 incident upon the charge transport layer 3 reaching the surface of the metallic conductive support 1, reflected there and exiting the surface of the charge transport layer 3.
Assuming that the refractive index of the laminated charge generation layer 2 and charge transport layer 3 is n and the thickness of the laminate is d and the wavelength of laser beam is .lambda., when nd is a whole number multiple of .lambda./2, the intensity of reflected light is maximum, that is, the intensity of the light entering the charge transport layer is minimum (according to law of conservation of energy), while when nd is an odd number multiple of .lambda./2, the intensity of reflected light is minimum, that is, the light entering the charge transport layer is maximum. It is inevitable due to the accuracy in the fabrication that d has a fluctuation of thickness of about 0.2 .mu.m. On the other hand, it is considered that the laser beam is so monochromatic and coherent that the condition for interference changes corresponding to the irregularity of the thickness d. The quantity of laser beam absorbed in the charge generation layer is different from place to place results in irregularity of density of solid image in a form an of interference fringe. When ordinary copying machines are used, the width of irregularity of density in form an of interference fringe is varied depending upon the wavelength and is an average value because the light source is not monochromic, and thereby the irregularity of density is not seen.
Heretofore, in electrophotographic methods using laser beam, generation of irregular density in a form of interference fringe has been prevented, for example, by roughening the reflecting surface of a substrate and the interface between the ground layer and the photosensitive layer to form unevenness so as to cause phase difference of the reflected light beams. However, such surface roughening makes the photosensitive layer formed on the uneven surface nonuniform in the case of a laminate type photosensitive member and therefore there are caused disadvantages such as defective images and remarkably lowered photographic characteristics.